1. Field of the Invention
The present invention relates generally to chemical mechanical planarization (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to enhancing the performance of metal CMP systems.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform CMP operations, including planarization, buffing and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In some other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization. Copper (Cu) CMP became a Cu Dual Damascene technology enabling operation as none of the other technologies are capable of shaping the copper plugs and wires.
CMP systems typically implement rotary, belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of a wafer. Normally, the removal of excess dielectric and metallization layers is achieved through in situ chemical modification of the processed wafer surface thus making the wafer surface more pliable for material removal. Slurry is used to facilitate and enhance the chemical modification of the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., pad, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The slurry distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface. Subsequently, the chemically modified excess metallization and dielectric layers are removed from the surface of the wafer. As the metallization and dielectric layers each have different chemical characteristics, the chemical mechanical planarization operation of the metallization layers defer from chemical mechanical planarization operation of dielectric layers. The following is a brief description of both CMP operations.
The chemical mechanical planarization operation of dielectric layers are achieved by dissolving the dielectric layer (i.e., the oxide layer) in hot water under pressure so as to create loose polyhydrosilicates. Thereafter, the polyhydrosilicates can be removed from the wafer surface easily. In contrast, the chemical mechanical planarization operation of metallization layers poses a significant challenge, as the ductile characteristic of metals renders excess metallization layer removal almost impossible. In contrast to non-metal elements in which electrons are localized between the atoms, the valence electrons of metal elements are not localized between a pair of atoms and create a “conductivity zone,” “electron cloud,” or “electron layer.” As a result, the free metal ions are attracted to the electron layer. Thus, the resulting metal atoms can be easily moved along the surface of the metallization layer without causing the bond between them and the electron cloud to break. Frequently, this is related to the “ductile” nature of metals, which herein is referred to as the ability of the attached resulting metal atoms to easily move from their respective equilibrium positions on the surface of the metallization layer without breaking the metallic bond between them and the surface of the metallization layer. As a comparison, in non-metal elements having localized electrons, normally, the molecular bonding can be easily broken by simply changing the angles of the atoms by 20% to 30%.
As such, to perform CMP operation on the metallization layers, the metallization layers must be converted into chemical compounds having molecular bonding (e.g., oxides, etc.). In another word, the metallic bonding between the resulting metal atoms and the metallization layer must be changed into a form of molecular bonding wherein electrons are localized between two specific atoms. Thus, in metal CMP operations, metallization layers are oxidized, thereby creating oxidized layers. As the oxide molecules have molecular bonding, the oxidized layers can be easily removed mechanically.
An exemplary prior art CMP system 100 is shown in FIG. 1. The CMP system 100 of FIG. 1 is a belt-type system, so designated because the preparation surface is an endless polishing pad 108 mounted on two drums 114 which drive the polishing pad 108 in a rotational motion as indicated by polishing pad rotation directional arrows 116. A wafer 102 is mounted on a carrier 104. The carrier 104 is rotated in direction 106. The rotating wafer 102 is then applied against the rotating polishing pad 108 with a force F to accomplish a CMP process. Some CMP processes require significant force F to be applied. A platen 112 is provided to stabilize the polishing pad 108 and to provide a solid surface onto which to apply the wafer 102. Depending on the type of excess material being removed, a slurry 118 including of an aqueous solution such as NH4OH or DI water containing dispersed abrasive particles is introduced upstream of the wafer 102.
As in metal CMP operations the metallic layer must first be oxidized, the composition of slurry 118 is an important aspect of the metal CMP operations. In addition, the utilized slurry 118 must be chosen such that the slurry 118 would not induce corrosion and imperfections onto the wafer surface 102. As such, typical metal CMP slurries contain oxidizers and acids, each of which facilitates the conversion of metallization layers into oxide layers. Conventionally, these slurries are designed to be “highly stable” and as such have two basic characteristics. First, they have a sufficiently long shelf lifetime. Second, they require a significantly high amount of energy to be activated. As such, the highly stable characteristic of metal slurries yields a significantly low oxidation rate, hence reducing the overall removal rate. As a result, the overall time expended in the metal CMP process is significantly increased thereby reducing the throughput.
In view of the foregoing, a need therefore exists in the art for an enhanced chemical mechanical planarization system that yields a higher throughput utilizing conventional slurries.